1. Field of the Invention
The present invention relates to a dispersion compensation type optical signal receiving apparatus used for optical communications and, more specifically, to a dispersion compensation type optical signal receiving apparatus, a receiving circuit, a receiving method, and a receiving program, which are capable of avoiding saturation of a limiter amplifier circuit.
2. Description of the Related Art
The refractive index of the core of the optical fibers differs depending on the wavelength, so that light is propagated at a different speed in accordance with its wavelength. This phenomenon is called dispersion, and the reaching time of the light to a receiver differs for each wavelength because of this phenomenon. Thus, the waveform of an optical signal at the input of the receiver is distorted, and the pulse width is widened, thereby deteriorating the receiving quality. In order to compensate the dispersion optically, it is common to offset the dispersion by adding a dispersion compensation fiber that has a reversed characteristic with respect to that of the optical fiber of that transmission line. However, recently, an electronic dispersion compensation (EDC) IC, which performs dispersion processing after converting an optical signal to an electric signal, has been put into practical use, and it has become possible to compensate the dispersion economically.
FIG. 4 shows a circuit block diagram of a conventional optical receiver. Further, FIG. 5 shows a circuit block diagram of an optical receiver that includes a conventional electronic dispersion compensation (referred to as EDC hereinafter) IC. In FIG. 4 and FIG. 5, reference numeral 102 is a photocurrent monitoring circuit, 103 is an APD (Avalanche Photodiode) element, 104A (204A) is an preamplifier circuit, and 104B (204B) is a limit amplifier circuit.
Further, reference numeral 106 (206) indicates an APD module that is constituted with the APD element 103, the preamplifier circuit 104A, and the limit amplifier circuit 104B. Reference numeral 107 is an EDC (electronic dispersion compensation) IC, 108 is a clock/data reproducing circuit (referred to as CDR hereinafter), and 204B is an AGC (Automatic Gain Control) amplifier circuit.
An optical input signal propagated through the optical fiber is photoelectric-converted into a current signal at the APD element 103, which is then converted to a voltage signal at the preamplifier circuit 104A. Then, it is amplified at the limit amplifier circuit 104B, which is then inputted to the CDR (clock/data reproducing circuit) 108 as a differential voltage signal to reproduce a clock and data.
As shown in FIG. 5, the EDC (electronic dispersion compensation) IC 107 is provided between the output of the APD module 206 and the input of the CDR 108. As described above, the EDC (electronic dispersion compensation) IC 107 is an IC for correcting the waveform distortion caused due to the pattern effect that is generated because the signal light transmitted through the optical fiber for a long distance is propagated at different speeds depending on the wavelength thereof. This EDC IC 107 serves a function of correcting the influence of several bits before and after a certain bit. Thus, the information regarding how the waveform is distorted after the long-distance transmission is important.
The limit amplifier circuit 104B has a certain gain and serves a function of amplifying the voltage signal. However, when the input signal light becomes more than a certain power, the output amplitude of the limit amplifier circuit 104B becomes saturated to be a certain amplitude. Thus, in an area where the output amplitude is saturated, the output waveform becomes different from the input waveform and the information of the waveform affected by the pattern effect after the long-distance transmission is lost. Thus, the EDC IC 107 does not function properly. Therefore, the effect of the EDC IC 107 cannot be utilized fully, and the CDR (clock/data reproducing circuit) 108 provided thereafter cannot reproduce the clock and data accurately.
Thus, when using the EDC, as shown in FIG. 5, it is common to use the AGC amplifier circuit 204B instead of the limit amplifier circuit 104B to decrease the gain when the input power is large, so that the output amplitude is not saturated. It is possible with the use of the AGC amplifier circuit 204B to control the gain to be decreased when the input level is high, and to be increased when the input level is low. Therefore, even if the input light level is high, the input waveform can be amplified in that form and the waveform distortion after the long-distance transmission can be maintained. Thus, the EDC can exhibit the effect thereof.
However, it is necessary for the AGC amplifier circuit 204B to be designed in such a manner that there is no change in the band even if there is a change in the gain. Practically, the AGC amplifier circuit 204B that operates at 10 Gbps is still not very common, and it is hard to expect when the circuit of 40 Gbps can be achieved.
Conventionally, there is reported a light receiving circuit that comprises a variable optical attenuator in the middle of an input line of an optical receiver for an optical input signal, in which the attenuation amount of the variable optical attenuator is changed in accordance with the detection result of a bias current of the light receiving circuit (Japanese Unexamined Patent Publication 04-275468 (Patent Literature 1)).
However, the light receiving circuit disclosed in Patent Literature 1 is for attenuating the input signal light so that the signal light of more than a threshold value (the maximum overload) that can be received by the light receiving circuit cannot be inputted. Thus, in terms of the object and the effect, it is different from the receiver of the present invention which allows the electronic dispersion compensation function to be fully utilized by limiting the signal light power that can be received without any trouble by a conventional receiver whose limit amplifier provided therein is saturated.